I regressed to the 15.11 drivers.  The newer driver wouldn't keep my GPU overclock speeds after alt-tabbing or exiting the game a few times.  The older drivers work fine with it.
With both the newest driver and the 15.11, even at 1920x1080, the same thing happened.
I reset the CPU to the default multiplier (33) and had these results:
Res: 1440p, 59fps in the game, GPU 1108/1250, CPU 1.60GHz (3.30GHz)
First run:  2-3 fps drop when vehicle spawns enabled
Second run:  game froze, and the screen ended up white.  I had to end the AcesHigh process.
Third run: 2-3 fps drop, same as first run.
I then went back to BIOS, and set the CPU multiplier to 39 (so now, 3.9GHz).
When I tested again, adding the vehicle spawns dropped the fps to 18-20.
So, now we know there's something linked to the higher OC on the CPU.  Max temps under load, even with the OC are only 51-53c per core.
What's the next step?  It doesn't make sense to me.